How much gas does a parked car running use? In this case a Honda Accord

Hey guys I have a Honda Accord 2011. Don't know too much about cars. How much gas does it use while on and parked, relative to when on and driving in the city? Like about half as much?How long would it take to use up one gallon of gas id it's on and parked? via /r/cars http://ift.tt/2s1FQYr